Thryomanes bewickii

Might be conspecific with and may constitute a superspecies with T. sissonii (AOU 1998).

Widespread and common in many areas in western North America; major declines, probably related to habitat loss and habitat succession, have occurred east of the Mississippi River.

BREEDING: historically, mainly from southwestern British Columbia, western and central Washington, western and southern Oregon, northern California, west-central and southern Nevada, southern Utah, southwestern Wyoming, central Colorado, Kansas, southeastern Nebraska, southern Iowa, extreme southern Great Lakes region, and southeastern New York, south to southern Baja California, northern Sonora, in Mexican highlands to central Oaxaca, western Puebla, and west-central Veracruz, and to southern Tamaulipas, central Texas, northern Arkansas, northern portions of the Gulf states (except Louisiana), and central South Carolina (AOU 1983). Formerly occurred on San Clemente Island (California) and Isla Guadalupe (Mexico). Species has nearly disappeared from all of the range east of the Mississippi River (Kennedy and White 1997). The greatest numbers are now, and probably always have been found, in the southwestern U.S. The highest encounter rates from 1976 to 1985 were found in central and southern Texas, central and southern Arizona, and southern and northcentral California. Highest average birds per BBS routes (1982-1991) were located in Arizona (9.67), Texas (5.24), California (4.25), New Mexico (3.37), and Oklahoma (2.57). A small population concentration also exists in western Washington. Eastern states, in contrast, average 0.05 birds per route. NON-BREEDING: northern limits of breeding range (west of the Rockies), Kansas, Missouri, lower Ohio Valley, Tennessee, and North Carolina south to limits of breeding range in Mexico, the Gulf coast, and central Florida (AOU 1983). The population east of the Mississippi has nearly disappeared. Subspecies ALTUS: historically, Appalachian region from southern Ontario, central Ohio, and cental Pennsylvania south to central Alabama, central Georgia, and central South Carolina, wintering south to the Gulf coast and central Florida (AOU 1957); now very local and rare in this range.

Numerous occurrences, especially in the southwestern U.S.

Threats are poorly defined, but eastern populations are clearly threatened, and possibly western populations as well. Declines may be due to interspecific competition, habitat changes, inclement weather, and predators. Interspecific competition has been strongly linked to the decline. Starlings (STURNUS VULGARIS), house sparrows (PASSER DOMESTICUS), house wrens (TROGLODYTES AEDON), Carolina Wrens (THRYOTHORUS LUDOVICIANUS), and Song Sparrows (MELOSPIZA MELODIA) are likely competitors although it is difficult to attribute decline entirely to competition with any one species (Byrd and Johnston 1991, Ehrlich et al. 1992, LeGrand 1990, Simpson 1978). In Tennessee, for example, the decline began prior to the arrival of House Wrens (Hamel 1992). It is also difficult to ascertain what limiting resources are involved in the competitive exclusion of this species. Nest sites (cavities and crannies in tree trunks, base of trees and shrubs, or in buildings), for example, are not likely to be in short supply. Insects, the wren's food source, are seldom a limiting factor in the breeding season in the eastern U. S. (LeGrand and Hall, unpubl. data). Byrd and Johnston (1991) suggest that modern suburbanization and forest regrowth may be the most likely reason for the population decline. The San Clemente and Guadalupe Island subspecies, for example, are extinct due to habitat destruction caused by introduced livestock. Additional factors in the decline may be related to a series of harsh winters in 1957 and during the late 1970's (Mengel 1965, Robbins et al 1986, Peterjohn 1989). Bent (1948) noted also a few instances of brown-headed cowbird (MOLOTHRUS ATER) and bronzed cowbird (MOLOTHRUS AENEUS) parasitization. Predators include hawks, owls, and snakes. Pesticides are not known to be a problem, but it is unclear whether or not this has ever been directly investigated. Threats in the far West, where the species may also be declining, have not been analyzed.